ATLANTA, Georgia (CNN) -- "Cloverfield" is the first adrenaline-pumping monster hit of the year, bringing in more than $40 million dollars on its opening weekend. The thriller is told from the point of view of five young New Yorkers using their handheld camera. But for some viewers, being "part" of the movie is making them sick -- literally.

One blogger on the popular movie database IMDB.com said, "I had to get up and leave the theater for nearly 20 minutes just to keep from hurling." Other moviegoers have reported being nauseated and dizzy.

Most viewers are unaffected by the film, but for those who are, experts say the problem is in their heads.

"This is a classic case of vertigo," said Dr. Michael G. Stewart, chairman of otorhinolaryngology (ear, nose and throat medicine) at NewYork-Presbyterian Hospital/Weil Cornell Medical Center. "You can look around and feel like things are moving, when they aren't."

Vertigo is caused when a person's balance system gets confused. Your body feels a strong visual sensation of movement but in reality, you aren't moving at all. The disconnect confuses your brain sensors and can cause dizziness and nausea, Stewart said. Watch Dr. Gupta explain what's making viewers sick »

AMC theaters across the country have posted signs warning moviegoers of potential "side effects associated with motion sickness similar to riding a roller-coaster."

Don't Miss

"People have different levels of susceptibility, similar to how some people cannot ride on a small boat without getting sick," Stewart said. "It's just a natural variation."

The good news is, experiencing vertigo is rare for most people. So if you are itching to see "Cloverfield" but are worried you might get sick, experts recommend taking a dose of over-the-counter anti-vertigo medicine, sold under such brand names as Bonine and Dramamine II. "It might not protect you from all the symptoms, but it could, and it certainly can't hurt," Stewart said.

Health Library

MayoClinic.com: Dizziness

Another tip is to briefly close your eyes during the movie. It may be enough to recalibrate your sensors and soothe your symptoms long enough to finish the movie.

And if you're really concerned, just wait to rent the movie.

"A person would be fine watching from home," Stewart said. "When you are on your couch, you have perspective around the screen so your mind knows the movie is moving and the room isn't."E-mail to a friend